The present invention relates to a shadow mask type color cathode ray tube, and more particularly to a color cathode ray tube having a shadow mask provided with press-formed embossments in a skirt portion thereof of such dimensions that concentration of stress in an apertured portion of the shadow mask is prevented in the operation of fitting the skirt portion of the shadow mask into a support frame.
In general, a shadow mask for use in a color cathode ray tube is press-formed and it has an apertured portion having a multiplicity of electron-transmissive apertures, an imperforate portion surrounding and integral with the aperture portion and a skirt portion bent back from a periphery of the imperforate portion. The skirt portion of the shadow mask is fitted in a support frame, is spot-welded to the support frame and fixed in a panel portion of the color cathode ray tube such that the imperforate portion of the shadow mask faces a phosphor screen coated on an inner surface of a faceplate of a panel portion.
FIGS. 4A to 4C are respectively structural views showing an example of the shadow mask used in a conventional color cathode ray tube. FIG. 4A is a front view of the shadow mask, FIG. 4B is a side view of a short side of the shadow mask, and FIG. 4C is a fragmentary sectional view of a region extending from the imperforate portion to the skirt portion.
In FIGS. 4A to 4C, reference numeral 40 designates a shadow mask, 41 is an apertured portion, 42 is an imperforate portion, 43 is a skirt portion, and x marks are weld points.
The shadow mask 40 has a curved apertured portion 41 having a multiplicity of electron-transmissive apertures (not shown), a curved imperforate portion 42 surrounding and integral with the apertured portion 41 and a skirt portion 43 bent back from a periphery of the curved imperforate portion 42 , and is usually integrally formed by press-forming a multi-apertured thin sheet-like metal blank.
The multi-apertured thin sheet-like metal blank is very thin and therefore the press-formed shadow mask 41 is not always good in forming characteristics. The strength of the thin sheet-like metal blank is relatively weak and the shape of the shadow mask 40 obtainable by press-forming is limited. The corners of the shadow mask 40 are bent back from the curved imperforate portion 42 to form the skirt portion 43 with a smoothly falling curve, or are bent back stepwise from the curved imperforate portion 42 to form the skirt portion 43 with at least one step. The portion between the corners of the shadow mask 40 is bent back from the curved imperforate portion 42 with a relatively small radius to form the skirt portion 43. As a result, the skirt portion 43 of the shadow mask 40 curls outwardly by a distance AS from a straight line passing through a bend line between the imperforate portion 42 and the skirt portion 43 and parallel to the longitudinal axis of the cathode ray tube, in a region between the corners of the shadow mask 40, as shown in FIG. 4C.
The fixation of the press-formed shadow mask 40 to a support frame (not shown) is performed as follows: the skirt portion 43 of the shadow mask 40 is fitted inside the support frame (rarely outside the support frame), and is spot-welded to the support frame at several weld points marked with X as shown in FIG. 4B. The number of the weld points of the skirt portion 43 and the support frame are two on each long side, two on each short side and one at each corner of the shadow mask 40, for example.
Large curl .DELTA.S is always easily made in the skirt portion 43 in press-forming the above-explained conventional shadow mask 40. If the size of the curl A S exceeds an acceptable limit, the problem arises in that the curl .DELTA.S is an obstacle to fitting the skirt portion 43 into the support frame and degrades workability of the fitting operation. If the skirt portion 43 having a large curl .DELTA.S is forcibly fitted into the support frame, the stress caused to the skirt portion 43 is transmitted to the imperforate portion 42 and the apertured portion 41, distorts the curved contour of the apertured portion 41 of the shadow mask 40, and as a result, the color selection property of the shadow mask 40 is degraded.
To solve these problems, Japanese Patent Application No. Hei 9-56286 filed (laid-open on Sep. 25, 1998) by the same applicants as the present application, which corresponds to the copending application Ser. No. 09/035,896 which issued as U.S. Pat. No. 6,111,346, proposes a color cathode ray tube which is provided with a plurality of embossments and a plurality of notches around the circumference of the long and short sides of the skirt portion 43 of the shadow mask 40 such that the embossments extend in a direction of the height of the skirt portion 43 and project arcuately toward the imperforate portion 42, and such that the notches are disposed between two adjacent ones of the embossments 43 and extend a fraction of the height of the skirt portion 43 in the direction of the height of the skirt portion, in order to limit the size of curls .DELTA.S occurring in the skirt portion 43, the embossments and notches being formed in the operation of press-forming the shadow mask 40.
In the above proposal, the plural embossments and notches formed in the skirt portion 43 limit the size of the curls .DELTA.S occurring in the skirt portion 43 such that concentration of stress in an apertured portion 41 of the shadow mask 40 is prevented in the operation of fitting the skirt portion 43 of the shadow mask 40 into a support frame.